Radioisotopic Space Power--Prospects and Limitations Page: 15 of 26
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For the Pm214703 Case:
Inside volume is 106 ccm with no need for void space allowance.
106 x 1.8 watts/ccm (power density of Pm214703)
Heat output of capsule = 190.8 watts - 1.46 watts/ccm of total capsule
volume (external).
This comparison shows that promethium oxide in the encapsulated form may have
a significantly higher practical power density than does plutonium-238 oxide.
This fact is not apparent from a cursory inspection of radioisotope pro-
perties (Table I) but comes to light when the materials are considered from
the standpoint of practical engineering design.
For smaller capsules, the comparison, as made above, would be even more
divergent. This situation should be taken into account when comparing any
of the alpha emitters with the beta emitters.
A similar situation exists in the case of the isotopes of very high power
densities such as Po-210. In such materials it is unlikely that the pure
isotope could be handled, fabricated or even used without some dilution.
The heat output from even small volumes of the pure materials is so high
that extraordinary means are needed to prevent melting and volatilization of
the isotopes as well as the encapsulating materials. Thus, from a practical
standpoint, designs could probably employ Po-210, Cm-242 or Th-228 only with
substantial dilution and thus at more modest power densities.
A comparison of shielding requirements is shown in Table II. Based on
these data, Pm-14+7 and Pu-238 require much less shielding than the other
isotopes.
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Sayre, E. D. & Rohrmann, C. A. Radioisotopic Space Power--Prospects and Limitations, report, March 13, 1964; Richland, Washington. (https://digital.library.unt.edu/ark:/67531/metadc1027900/m1/15/?rotate=270: accessed May 7, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.